The present invention relates to a high pressure discharge lamp driving apparatus, a luminaire and high pressure discharge lamp driving method.
PCT application Japanese laid-open patent Toku-Hyo-Hei 10-501919 (PCT/ID95/00392) discloses an apparatus and method for driving a high pressure discharge lamp with an AC lamp current, wherein a pulse current is generated in a predetermined fraction of a half-cycle period of lamp current, and the polarity of the pulse current is made the same as the lamp current, then the pulse current is superposed on the lamp current in the second half of each half-cycle period where it is generated.
It is also described that good results may be achieved when the ratio between the mean amplitudes of the pulse current and the lamp current is in the range of 0.6 to 2, and the ratio between the duration of the pulse current and the half-cycle period of the lamp current is in the range of 0.05 to 0.15.
In the prior art as described above, since the total amount of current flowing through the high pressure discharge lamp increases at the end of a predetermined fraction of the half-cycle period of the lamp current by the pulse current, the electrode temperature rises to a relatively higher value. The high temperature enhances the stability of a discharge arc because the discharge arc originates from the same place on the electrode in each cathodic phase.
However, in the case of prior art, when the pulse current flows, the current temperature rises relatively high only for a short time, so that an emitted light also increases remarkably. Accordingly, the prior art has a drawback that a rapid change of the output amounts of light by superposing the pulse current on the lamp current causes flickering of color or brightness on a picture even if for a short time, in case of using a light emitted for extremely short time such as the digital light processing (trade name; xe2x80x9cDLPxe2x80x9d) system which obtains a color picture by superposing the RGB three-primary-color picture by timesharing and then projecting the picture by utilizing a revolving RGB color filter and a digital micro-mirror device (trade name; xe2x80x9cDMDxe2x80x9d).
Therefore, the prior art could not achieve an operation or effect of reducing the flickering of the output amounts of light in regard to a usage pattern of the output amounts of light.
Also, in the prior art, an electrode substance tends to be exhausted by sputtering when the electrode is heated by superposed on the pulse current. Therefore, the prior art has another drawback that since the inner surface of a translucent discharge bulb is blackened to decrease the transparency of the bulb, the life of the high pressure discharge lamp is shortened as the output amounts of light decreases.
An object of the present invention is to provide a high pressure discharge lamp driving method, a high pressure discharge lamp driving apparatus, and a luminaire utilizing the high pressure discharge lamp driving apparatus, which decrease the a fluctuation of emitted light in half-cycle period of the lamp current, so as to control flickering of the output amounts of light.
According to a first aspect of the invention, there is provided a high pressure discharge lamp driving apparatus for driving a high pressure discharge lamp during a stable operation thereof with an AC lamp current alternating for opposite half-cycle periods, the apparatus comprising:
an AC lamp current supply for supplying the AC lamp current to the high pressure discharge lamp;
a detector for detecting the output AC lamp current to the high pressure discharge lamp;
a controller responsive to the detector for controlling the AC supply to output the AC lamp current with a waveform which is characterized by that;
(i) a mean value in the second half of each half-cycle period is larger than the means value of the first half of each half-cycle period;
(ii) no pulse current is contained in the second half of each half-cycle period;
(iii) the AC lamp current has a maximum value immediately after polarity change between the positive and negative half-cycle periods; and
(iv) the AC lamp current has also a waveform defined by that the ratio (d/a) of the maximum value (d) of the AC lamp current and a mean value (a) of the AC lamp current in its each half-cycle period satisfies a following equation.
1.05xe2x89xa6d/axe2x89xa61.80 
In the first aspect of the invention, it is possible to heat the electrode fully to the very end part of the second half of the half-cycle period. As a result, it is possible to keep the electrode temperature high to a required level in the next half-cycle period after the polarity fluctuation of the lamp current. Accordingly, when an action of electrode has switched from an anodic phase to a cathodic phase, it is controlled that a cathode spot moves over the electrode surface, thus the stability of the discharge is enhanced.
Also the instantaneous fluctuation of the output amounts of light is remarkably decreased. Thus, even in case of using the high pressure discharge lamp that is operated according to the first aspect of the invention in the digital light processing system, flickering of brightness or color will be hardly produced.
Furthermore, it is hard to produce a sputtering of the electrode substance caused by the electrode pulse at about the end of half-cycle period, so that the life of the high pressure discharge lamp is improved.
According to the second aspect of the present invention, there is provided a luminaire comprising a lamp driving unit main body and a high pressure discharge lamp driving apparatus mounted on the luminaire main body, wherein the driving apparatus is configured in a manner as defined in the second aspect of the invention.
Additional objects and advantages of the present invention will be apparent to persons skilled in the art from a study of the following description and the accompanying drawings, which are hereby incorporated in and constitute a part of this specification.